1. Technical Field
The present invention relates to an oscillator.
2. Related Art
Generally, there is known an oscillator using a micro electro-mechanical system (MEMS) technology, and having an MEMS element provided to a semiconductor substrate. In such an oscillator, the MEMS element is used as a vibrator, and a clock pulse with a target frequency is output to an external circuit or the like.
As the MEMS element used for the vibrator of the oscillator, there is known an element having, for example, a fixed electrode and a movable electrode, the movable electrode vibrating in accordance with application of a specific alternating-current voltage between these electrodes (e.g., JP-A-2007-111831). Further, the specific alternating-current voltage generally has the natural resonant frequency (hereinafter referred to as a resonant frequency) of the movable electrode.
As such an oscillator, the oscillator shown in FIGS. 10A and 10B, for example, can be cited. The oscillator shown in FIGS. 10A and 10B is composed of an oscillation circuit composed of an MEMS vibrator and an active circuit, a bias voltage applying circuit for applying a bias voltage to the MEMS vibrator, and a buffer circuit for preventing the mutual interference between the oscillator and an external circuit.
The vibrator provided to such an oscillator has a characteristic that the resonant frequency varies when a static voltage (a bias voltage) applied between the movable electrode and the fixed electrode is varied. It should be noted that the bias voltage is generally a direct-current voltage. Therefore, it is possible to adjust the bias voltage to be applied, thereby adjusting the resonant frequency. Since such a vibrator is very tiny, it is difficult to manufacture the vibrator so as to have the resonant frequency falling within the target range with good accuracy. However, by appropriately adjusting the bias voltage, it is possible to make the resonant frequency fall within the target range.
Further, in the case in which the resonant frequency of the vibrator due to the bias voltage is sufficiently small with respect to the bias voltage which can be applied, the vibrator has the characteristic that the fluctuation range of the frequency increases as the bias voltage becomes higher. Therefore, in the case in which the frequency fluctuation range of the vibrator needs to be broadened, a high bias voltage might be intentionally applied to the vibrator.
Further, the appropriate bias voltage of the vibrator (the MEMS element) is determined in accordance with the shape and the form of the vibrator, and the voltage higher than the drive voltage of the active circuit of the oscillator might be required depending on the shape. Although in most cases the operation voltage of the active circuit of the oscillation circuit is set to be a relatively low value (e.g., 3V), in the case in which the appropriate value of the bias voltage exceeds the value (e.g., 5 through 10V), it becomes necessary to add a step-up circuit.
As the oscillator for making the resonant frequency fall within the target range, or for broadening the frequency fluctuation range of the vibrator, an oscillator having a configuration shown in FIG. 11, for example, can be cited. In the case of using such an oscillator, it becomes possible to control the oscillating frequency of the oscillation circuit by the bias voltage applied thereto.
The oscillator shown in FIG. 11 is composed of a reference voltage supply circuit for supplying the reference voltage, a step-up circuit for converting the reference voltage into a higher voltage, an oscillation circuit for applying the voltage thus stepped up, to the vibrator and obtaining clock pulses from the vibration of the vibrator to thereby output the clock pulse to the external circuit, and a buffer circuit for preventing the mutual interference between the oscillator and the external circuit.
As described above, the step-up circuit for stepping up the bias voltage might be used in order for applying the appropriate bias voltage, or for broadening the fluctuation range of the resonant frequency of the vibrator. As the step-up circuit, a charge pump circuit is preferably used. The charge pump circuit has a relatively small area, and can be incorporated on, for example, a semiconductor substrate, and is therefore suitable for miniaturization of the oscillator. However, the charge pump circuit operates by the clock pulses obtained from the outside. Therefore, in the case in which the clock pulses of an external quartz oscillator or the like are used in the charge pump circuit, since the clock pulses of the quartz oscillator or the like do not necessarily have a frequency equal to that of the oscillator or the vibrator (the MEMS element), there is concern that the problem of deterioration (e.g., increase in noise) of the clock pulses transmitted from the oscillator due to the interference arises. If the clock pulses transmitted from the oscillator are thus deteriorated, failure might be caused in the external circuit such as a microcomputer or a controller using the clock pulses output from the oscillator.